Art of reducing magnetostrictive effects in magnetic materials



March 18, 1941. HAYES 5 AL 2,234,968

ART OF REDUCING MAGNETOSTRICTIVE EFFECTS IN MAGNETIC MATERIALS Filed Nov. 12, 1938 f/v Use \SUQFACE AB/EAS/ON Nae/mu. 75 PQEDETE/{M/NED DIRECTION OF FLux Flow INVENTORS. '4 DAN I14. HA YES BY ND .Da/v S. Wen/ 0130.

ATTORNEYS.

Patented Mar. 18, 1941 UNITED STATES PATENT OFFICE ART OF REDUCING MAGNETOSTRICTIVE EFFECTS IN MAGNETIC MATERIALS Application November 12, 1938, Serial No. 240,202

14 Claims.

The phenomenon of magnetostriction, which is the property possessed by many magnetic materials of undergoing a dimensional change or distortion when magnetized and de-magnetized is well known. It becomes a significant factor in the construction particularly of electrical power transformers, since the magnetostriction of the core is an important source of the noise emanating from these transformers when in operation. The noise becomes so great in some instances as to constitute a public nuisance. As a consequence, it is desired to provide magnetic materials for this and for analogous uses, in which materials the magnetostrictive effect is at a minimum. It has come to be the practice to purchase magnetic materials to be used for such P rposes on specifications, one feature of which is a maximum permissible magnetostrictive effect. Different magnetic materials have different magnetostrictive characters; but it will frequently be found that different portions of the same batch of sheets exhibit different magnetostrictive effects. The cause of this variation in magnetostriction is not fully understood. A great deal of work has been done on various ways of minimizing magnetostriction, but so far as we know, without significant result.

The improvements in the magnetic properties of core loss and permeability, which have been made from time to time have made possible considerable economies in transformer design which involve the operation of the transformers at higher and higher induction. The magnitude of the magnetostrictive effect, however, increases with the induction, and further progress in this direction is blocked by its rising to a point where the transformers become objectionably noisy. In many cases it is necessary, because of this effeet, to design transformers to operate at lower inductions than the induction at which they should operate to take full advantage of the otherwise excellent magnetic properties of the core material. This lowered induction makes it necessary to use more material in the core, which increases its cost and size. Removal of this magnetostriction limitation is obviously desirable, not only to reduce the noise from transformers of present design, but to permit the complete utilization of the properties of present and future core material.

Various tests for magnetostriction and for the effect of magnetostriction in transformer operation have been worked out, but it is not felt that these need be described here.

The noise level in a transformer is usually Pz=Power in a standard comparison wave of low intensity The power in a sound wave is proportional to the square of the amplitude (a) of the wave.

P=K (a) (2) If in addition it be assumed that the amplitude of the sound wave is proportional to the magnetostriction (AL/L) one obtains for the power Where AL/L represents the ratio of the change in length to the original length, i. e., the magnetostriction. Substituting back in Equation (1) one can obtain By measuring the noise level of a transformer, and the magnetostriction of the steel in that transformer, application of the above relation will give the noise level of the transformer for all possible magnetostriction values.

On one batch of transformers recently tested for magnetostriction and noise level, the magnetostrictive effect was found to have a value of 500 10 which was representative of a noise level value of 90 decibels. Such a transformer therefore produced a noise level equivalent to that of the average motor truck at a distance of fifteen to twenty feet.

The object of our invention is the provision of a means of treating electrical steels to diminish their magnetostrictive effect, and the production of a new material as so treated. It is an object of our invention to provide a method of treating magnetic material which does not add greatly to the cost of it and which may be practiced upon the material either in the form of sheets or strips or in the form of completed transformer punchings. It is an object of our invention to provide a treatment which not only may be applied to material inthe course of manufacture so as to deliver a finished product having a minimum of magnetostriction for the particular analysis involved, but also which may be applied to a series of stampings found by test to have too high a magnetostrictive effect so as to salvage such materials when otherwise they would have to be discarded.

db. 20 log;

These and other objects of our invention which will be understood by the skilled worker in the art more fully upon reading these specifications, we accomplish by that certain procedure and in that certain novel product of which we shall now describe an exemplary embodiment.

This embodiment has to do with silicon steels, and it may be stated at the outset that it is effective to reduce the magnetostrictive characteristics of silicon steels of all analyses, including both alloys of which the principal ingredients are iron and silicon, and also alloys containing other materials, in addition to iron and silicon, such for example, as manganese or other alloying elements. But the invention herein involved is not limited to alloys classifiable as silicon steels, and is applicable to the general range of magnetic alloys of all types and will produce in them significant diminutions in magnetostriction. It will be understood that as the normal magnetostrictive effects of alloys will vary with their formulae, so also the procedure and treatment herein set forth will effect difierent diminutions in magnetostriction in different alloys, depending upon their formulae.

The basis of the invention here involved is the discovery of the peculiar action of sheets or laminae of magnetic alloys when their surfaces are abraded. Considering first transformer laminations in the form of elongated strips which are to be so located in a transformer coil that the lines of magnetic flux will pass through the laminae from one end to the other in a direction parallel to their longer or major axes, certain effects can be produced.- Others have observed that if the surfaces of the laminae are slightly abraded in the direction of the major axis the magnetostrictive effect exhibited by the laminae will be substantially increased. We have found that if the abrading is done in a direction substantially at an angle of 45 to the major axis of the strip, there will be little or no effect on its magnetostriction. We have made the important discovery, however, that if the abrading is done transversely to the major axis of the strip, i. e. across the strip and perpendicularly to the direction of flow of the magnetic lines of force therein, a notable decrease in the magnetostrictive effect will be observed upon the test.

The particular reasons why this is so are not completely understood. It is supposed that the magnetostrictive effect is one involving a surface condition so that if the surface is broken up in a direction transverse to the direction of primary distortion due to the magnetostrictive eifect, the resultant distortion will be considerably diminished.

In the drawing we have shown diagrammatically the action of abrading the surface of a lamina in a direction normal to a predetermineddirection of flux flow therein as said lamina is to be used.

The abrasion may be carried on in a number of different ways. It may be accomplished by hand by fastening a sheet of suitable abrading material such as emery cloth to a block of wood, positioning the laminae upon a table or other support and rubbing their major surfaces with the abrader block in a direction across the proposed lines of magnetic force, and perpendicular to these proposed lines for the greatest effect,

. as has been set forth; or a stone or other abrasive block may be employed. Likewise a grinding wheel may be set up so as to traverse the work in the proper direction, the direction of the rotation of the grinding wheel also being such as to abrade the work in the proper direction.

Sand blasting likewise will serve and is in general a relatively inexpensive and convenient way of carrying on the invention. In using the sand blast, having ascertained the direction in which the magnetic lines of force are to flow in the work under treatment, we direct the sand blast nozzle or nozzles onto the work at an angle (for example to 45) to the plane of its surface, the nozzle being directed across the work in a direction perpendicular to the proposed direction of the lines of force. To make this clearer, if the work occupies a horizontal plane, the axis of the nozzle will lie in a vertical plane erected perpendicular to a line representing the proposed direction of the lines of force in the work, and the nozzle will be directed toward the work at an angle, for example of 30 to 45. More than one nozzle may be employed simultaneously, of course; and it is possible to treat the whole surface of a large sheet, or a large section of a strip of magnetic material at one time.

Transformer stampings are preferably made in such a way that the lines of magnetic force passing through them (when theyare assembled in the form of a core) will coincide with the direction in which the magnetic material has been rolled. This is especially advantageous with some of the more modern magnetic materials which exhibit a much greater permeability in the direction of rolling than in other directions. It will be obvious that if the manner in which the transformer stampings are to be cut from sheets or strip is known, the sheets or strip can be given our treatment prior to the formation of the laminae or stampings. Thus if straight transformer legs are to be,cut from sheet or strip, it will be usual to treat the sheet or strip after manufacture but before the formation of the stampings by abrading it in a direction perpendicular to its major axis. But the laminations themselves may be treated after fabrication, and it will be clear that where L-shaped, C-shaped, or E-shaped transformer stampings are being made it will be within the scope of our invention to abrade the body portion of these shapes in one direction and the leg portion or portions in another direction or directions, depending upon the path of the magnetic lines of force passing through them after the assembly of the transformer. Thus the two legs of an L-shaped lamination may be abraded in directions perpendicular to the axes of these legs, although this involves abrading one leg in a direction at right angles to the treatment of I the other leg.

Moreover, magnetic materials may be abraded at any stage in their manufacture which is not followed by some other stage eifective to destroy the abraded surface. For example, sheets or strips of magnetic materials may be given the abrasion treatment by the steel manufacturer before the sheets or strips are sold to the transformer manufacturer; and the transformer manufacturer may then cut up the material as by slitting, shearing or stamping, and anneal and/or insulate the laminae so formed without destroying the effect of the abrasion, providing the surfaces of the laminae are not so corroded or oxidized during annealing as to form a new and non-abraded surface. An amount of rolling or working treatment suiiicient to produce a new surface on the material, will destroy the eflect of previous abrasion, as will be readily understood. Where it is necessary to follow such practices or their equivalent, it will become necessary to re-abrade the pieces.

The abrasion need not remove any material quantity of the body metal. It seems unnecessary to point out that where we speak of abrasion we mean an abrasion of the surface of the body metal. If the material is covered with a layer of scale or other substance it is not effective merely to abrade the surface of such layer. By way of illustration of the effect of the procedure herein outlined, we append a table show ing the result of abrasion in different directions of four straight grain strips of silicon steel fifty centimeters long and three centimeters wide. The silicon steel was a material containing approximately 4.7 per cent silicon and .010 per cent carbon, balance substantially all iron. The abrasion was accomplished by hand with a coarse emery paper fastened to a wooden block. The strips were supported on a flat surface during abrasion and rubbed in the directions indicated in the table below.

In the course of our work we have ascertained that the effect upon magnetostriction of surface abrasion is dependent upon the relation between the direction of the abrasion and the direction of the magnetic flux. The effect is at a minimum when the abrasion direction is inclined at an angle of substantially 45 to the direction of the flux. A maximum increase in magnetostriction occurs when the direction of the abrasion coincides with the direction of the flux. A maximum decrease in magnetostriction occurs when the direction of the abrasion is directly across or at 90 to the direction of the flux. We have indicated that the amount of decrease will vary with the composition of the material. The maximum lowering of magnetostriction by abrasion which we have so far observed on any material is 220 10- at an induction of kilogausses. The average decrease is approximately half that value; but with most magnetic materials it will be clear that our treatment will bring the magnetostrictive effect to values such that the averages of lots will be not greater than lllOXllH at 15 kilogausses, which results in a material suitable for use in power transformers.

It will be understood that we are not restricted toany particular method of abrading the surfaces of magnetic materials since any of the usual methods of abrasion app ar to serve; and when we speak of the direction of abrasion we intend to be understood as meaning the formation of abrasion scratches in the direction indicated irrespective of the particular manner in which the operation is carried on. The eflect of the process appears to be dependent upon the formation of a considerable number of scratches or ridges and depressions in the surface of the metal which are longer than they are wide and which preponderantly lie in a direction transverse to the proposed direction of the lines of magnetic force in the material.

Modifications may therefore be made in our invention without departing from the spirit of it.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent, is:

1. A process of treating magnetic sheet-like materials for inductive uses which comprises ascertaining the direction of flow of magnetic lines of force therein in the use indicated and abrading at least one major surface of such materials in a direction substantially normal to the said direction of flow of magnetic lines of force in said materials.

2. A process of treating magnetic sheet-like materials for inductive uses which comprises ascertaining the direction in which magnetic lines of force will flow in such materials in the use indicated, and forming in at least one major surface of such materials a plurality of indentations of greater length than width, the preponderant direction of the length of such indentations being substantially normal to the said direction of flow of said magnetic lines of force.

3. A process of treating magnetic sheet-like materials for inductive uses which comprises ascertaining the direction of flow of lines of magnetic force in said materials in said use and then breaking up at least one major surface of the said materials by scratching said surface in a direction substantially normal to said direction of flow.

4. A process of treating magnetic sheet-like materials for inductive uses which comprises ascertaining the direction of flow of lines of magnetic force in said materials in such use and scratching a major surface of said materials in a direction substantially normal to said direction of flow.

5. Magnetic sheet-like material for inductive uses comprising a body of magnetic material adapted to be located in inductive apparatus so that the lines of magnetic force will flow therein in a certain direction, a major surface of said material being scratched in a direction substantially normal to said first mentioned direction.

6. Magnetic sheet-like material for inductive uses comprising a body of magnetic alloy having a major surface broken up by relative depressions and protuberances of greater length than width and preponderantly tending in the direc tion of their length normal to an ascertained primary direction of flow of magnetic lines of force therein.

7. A lamination for a transformer consisting of a body of magnetic material having a major surface abraded in a direction transverse to the direction of flow of lines of magnetic force through said body.

8. A transformer lamination consisting of a body of magnetic alloy having a major surface broken up into relative ridges and depressions extending in a direction substantially normal to the direction of flow of magnetic lines of force through said body.

9. A transformer lamination consisting of a body of magnetic material having a major axis coinciding with the rolling direction of said material and the direction of its greatest permeability, said material having a major surface characterized by abrasion lines substantially normal to the direction of rolling.

10. A transformer lamination consisting of an elongated body of silicon steel having major surfaces abraded in a direction substantially normal to the longitudinal axis of said body, the direction of rolling of said lamination and its direction of major magnetic permeability substantially coinciding with said longitudinal axis.

11. A transformer having a coil comprising a plurality of laminae assembled together, said laminae being bodies of magnetic material havnations which comprises forming and finishing a magnetic alloy in sheet like form, abrading a major surface of said magnetic alloy in a direction perpendicular to the direction of rolling thereof and cutting said alloy apart into transformer laminations in such a way as to cause the direction of abrasion to lie substantially normal to the path of magnetic. lines of force in said laminations, without destroying the abraded character of said surface.

14. A transformer lamination consisting of a body of magnetic material havingportions lying in different directions, which portions will be traversed by magnetic lines of force in directions coinciding with their longitudinal axes, each of said portions having a major surface characterized by abrasion scratches lying in a direction substantially normal to its major axis.

DAN M. HAYES. DON S. WOLFORD. 

